The control of crystal and precipitate particle size is very important in some circumstances, in particular in the pharmaceutical and agro-chemical industries in which the final product form of the active principal of interest is in the form of a fine powder. The manner in which an active principal behaves in a biological system depends upon many factors inter alia the size of the particle and the crystal form. Small particles may be made by processes such as milling, but such processes may have a detrimental effect on the material properties and may also produce a significant proportion of particles which are unsuitable for the desired use, for example, they may be too small or of an inappropriate shape. Such particles may undergo morphological alterations, leading to undesirable surface polymorphological transformation which in turn may give rise to the formation of amorphous structures. The particles may become highly charged which may also contribute to undermining flow-rates. Also, particles destined for use in aerosols may be compromised should they become highly charged. Crystallisation of crystals in the desired size range directly from solution would be desirable.
For many years it has been known to bring about crystallisation by mixing a solvent containing an active principal to be crystallised with an anti-solvent, so that after mixing the solution is supersaturated and crystallisation occurs. The mixing may occur in the presence of ultrasonic irradiation or in a different manner in which ultrasonic irradiation is not used eg fluid vortex mixing. The term “anti-solvent” means a fluid which promotes precipitation from the solvent of the active principal of interest (or of a precursor of the active principal). The anti-solvent may comprise a cold gas, or a fluid which promotes the precipitation via a chemical reaction, or which decreases the solubility of the active principal of interest in the solvent; it may the same liquid as the solvent but at a different temperature, or it may be a different liquid from the solvent.
EP 1144065 describes a system in which mixing of anti-solvent with solvent comprising an active principal to be crystallised is achieved by using a flow rate ratio of anti-solvent: solvent of up to 10:1 in the presence of ultrasonic irradiation in a continuous flow cell. It is described that a warm solvent is mixed with a cold miscible anti-solvent, although the actual temperature of the cold anti-solvent is not disclosed.
EP 1469938 describes a system in which the flow rate of mixing of anti-solvent with solvent comprising an active principal to be crystallised exceeds that of the solvent, at a flow rate ratio of up to 10:1, typically of from 2:1 up to 5:1. The mixing is carried out in the presence of ultrasonic radiation.
The prior art processes enable the production of crystals using flow rate ratios of anti-solvent: solvent that are generally lower than 20:1 (i.e. towards a flow rate ratio of 10:1 to as low as 1:1).